1. Field of the Invention
The present invention is related to a voltage conversion device capable of enhancing conversion efficiency, and more particularly, a voltage conversion device capable of automatically adjusting a charge pump output voltage under different switch-on resistors and different load currents, to maintain at a preset level.
2. Description of the Prior Art
A charge pump is often used in a booster circuit or a voltage multiplier circuit. For example, a prior art liquid crystal display (LCD) device can utilize a charge pump to raise an output voltage from a lower voltage source, to provide a higher operating voltage for drivers such as source drivers or gate drivers. As shown in FIG. 1 and FIG. 2, the charge pump can be seen as a dual-end element, for converting an input voltage Vi into a positive multiple output voltage Vo (FIG. 1) or a negative multiple output voltage Vo (FIG. 2).
The prior art provides many methods for implementing charge pumps and related circuits. For example, FIG. 3 illustrates a schematic diagram of a constant charge pump 300. The constant charge pump 300 includes a level shifter circuit 302 and a charge-exchange-control switch circuit 304. Clock signals CLK, XCK and control signals S1, S2 provided by the level shifter circuit 302 effectively drive the charge-exchange-control switch circuit 304, so that the constant charge pump 300 converts an input voltage Vi to an output voltage Vo accurately for voltage boosting or voltage multiplying. However, the constant charge pump 300 is only suitable for operating with a smaller load change. If the constant charge pump 300 is applied on a design that has a larger load change, under a low load condition, the efficiency of the charge pump 300 seriously decays, and the charge pump 300 might not be able to operate when the load is too large.
The prior art further provides a capacitor push-pull charge pump 400, as shown in FIG. 4. The capacitor push-pull charge pump 400 includes a level shifter circuit 402 and a charge-exchange-control switch circuit 404. The charge-exchange-control switch circuit 404 is the same as the charge-exchange-control switch circuit 304 in FIG. 3, while in the level shifter circuit 402, the output transistors of the level shifter circuit 302 is replaced by output capacitors. Under this condition, the capacitor push-pull charge pump 400 can adjust the amplitude of the clock control signals according to charge loads, so as to automatically reduce the transforming charges, in order to provide a higher efficiency. However, the clock signal level of the capacitor push-pull charge pump 400 cannot reach a full voltage, and the output voltage Vo is not stable and varies with the load.
In short, there is an equivalent resistor (switch-on resistor) when the charge pump switch is on. A load current passing through the switch-on resistor decreases the average of a direct current level of the output voltage, and the greater the switch-on resister is, the more the load current varies, and the more the average voltage decreases. If the switch rate of the charge pump is adjusted to restrain the drop of the average voltage, the output voltage Vo might be greater than the voltage requirement of the load circuit power source, and cause serious efficiency loss.
In order to solve the above-mentioned problems, a prior art charge pump can couple a voltage regulator to the output end, to generate the output voltage Vo as shown in FIG. 5 and FIG. 6. However, there are two defects in the charge pump: one is a voltage stabilization capacitor CL should be attached, another is the charge pump multiplies the input voltage to a very high output voltage VCC or VEE, then decreases the voltage with the voltage regulator, which loses efficiency.